The invention concerns a method and an apparatus for scanning specimens using an optical imaging system and a scanning stage, images of the specimen being acquired by means of a camera, and/or measurements on the specimen being made by means of an optical measurement device, at specimen points Xp, Yp.
The images or measurements performed at the specimen points are used for inspection and mensuration purposes. Inspection and mensuration systems with microscopes, with which faults and defects on wafers can be recognized and classified, are used in particular in semiconductor technology in the manufacture of integrated circuits on wafers. For that purpose, the wafers are displaced with the scanning stage in the X and Y directions. Conventionally, at each inspection or measurement location (specimen point), focusing occurs in the Z direction using a focusing system, and an image is acquired and/or a measurement performed. Focusing at the specimen points is necessary, among other reasons, in order to compensate for mechanical errors of the scanning stage in the Z direction.
The mechanical shortcomings of the scanning stage cause running errors upon displacement of the scanning stage. The running errors are brought about principally by dead weight and by the design of the bearings and guides of the scanning stage. Because of differing torques (resulting, for example, from different overhangs of the scanning stage), differing forces are caused in the bearings of the stage guidance system depending on the particular X-Y position of the scanning stage. These result in different inclinations of the scanning stage as a function of its X-Y position. The scanning stage therefore runs in inhomogeneous fashion upon displacement, i.e. the distance between the surface of the scanning stage and the optical system varies during displacement of the scanning stage.
Because of these running errors of the scanning stage, even in the case of flat specimen surfaces the image field is focused manually or with a focusing system so that a sharply focused image is always available for visual observation, for image acquisition with a camera, or for optical measurement purposes. A method of this kind with conventional focusing at the observation location and subsequent image acquisition or measurement is, however, disadvantageous for the requirements of rapid passes with many image acquisition or measurement points on the specimen, because of the large time expenditure. This is especially the case, of course, when a specimen needs to be completely scanned for a 100% inspection, for example in the case of a wafer in order to identify and classify defects or particles on the wafer surface.
U.S. Pat. No. 6,256,093 describes an on-the-fly automatic substrate defect classification procedure in which the wafer is scanned using an X-Y stage in order to locate defects. A laser light source illuminates the wafer in spot fashion during scanning. No focusing means are used here. The light scattered by the wafer is acquired using at least two separate detectors and evaluated in terms of various properties such as intensity, linearity, and asymmetry.
U.S. Pat. No. 6,172,349 discloses an automatically focusing, high-resolution microscope in which surfaces within the microscope image field differing from the surfaces on which measurements are performed are used for focusing on a wafer. For this, during a set-up phase an identification is made, within each microscope image field, of that surface portion at which, during displacement in the focusing direction, the intensity meets the criterion of maximum signal-to-noise ratio. When the wafer is subsequently inspected, only the light reflection from those identified surface portions within the respective microscope image field is used to focus the microscope. In order to increase wafer throughput, the microscope stage can be continuously displaced from one measurement location to another, and images can be acquired on the fly.